12.2: C

HEMICAL

CALCULA

TIONS

STANDARD

3.e. Students know how to

OBJECTIVES

12.2.1. Construct mole ratios from balanced chemical equations and apply these ratios in stoichiometric calculations

12.2.2. Calculate stoichiometric quantities from balanced chemical equations using units of moles, particles, and volumes of gases at STP

WRITING AND USING MOLE RATIOS

Mole Ratio: a conversion factor derived from the coefficients of a balanced chemical equation interpreted in terms of moles.

In chemical reactions, mole ratios are used to convert between moles of reactant and moles of product, between moles or reactants, or between moles of products.

SAMPLE PROBLEM 1

PRACTICE PROBLEM 2

IN EXCESS

“A substance in excess” means that you have more than enough of that substance. For example, if the recipe calls for 2 eggs, but I have a dozen, then eggs are in excess.

H2 3 mol NH3

2 mol H2

1 mol H2

2.0 g H2

17.0 g NH3

1 mol NH3

• Known: 5.00 g CaC2

• Unknown: g C2H2

• We have to go from g CaC2 mole CaC2 mole C2H2 g C2H2

• g C2H2 = 5.00 g CaC2 x ___________g CaC2

1 mol C2H2

1 mol CaC2

1 mol CaC2 26 g C2H2

1 mol C2H2

X ___________ X _________

Known = 49.0 g H2OUnknown = mole CaC2

We have to go from g H2O mol H2O mole CaC2

Mole CaC2 = 49.0 g H2O x __________

18.0 g H2O

1 mol H2O 1 mol CaC2

2 mol H2OX ___________

FIGURE

OTHER STOICHIOMETRIC CALCULATIONS

In a typical stoichiometric problem, the given quantity is first converted to moles.

Then the mole ratio from the balanced equation is used to calculate the number of moles of the wanted substance.

Finally, the moles are converted to any other unit of measurement related to the unit mole (depending on the problem).

CALCULATING MOLECULES OF A PRODUCT

#7

#8

VOLUME-VOLUME CALCULATIONS

PRACTICE PROBLEMS 10

PRACTICE PROBLEM 11